Electron beam irradiation apparatus and method

ABSTRACT

An electron beam irradiation apparatus is employed for irradiating combustion exhaust gas with an electron beam to remove toxic components from the exhaust gas. The electron beam irradiation apparatus comprises an electron beam source ( 12 ) for emitting electrons, an accelerating tube ( 13 ) for accelerating the electrons emitted from the electron beam source, a focusing electromagnet ( 16 ) for controlling a diameter of an electron beam by applying a magnetic field to an electron beam having a high energy formed in the accelerating tube ( 13 ), an electromagnet ( 17, 18 ) for deflecting and scanning the electron beam by applying a magnetic field to the electron beam and an irradiation window ( 20 ) for allowing the electron beam to pass therethrough. The electron beam is focused a focus point by the focusing electromagnet ( 16 ) so that the electron beam converges once and then diverges, and then emitted through the irradiation window ( 20 ) to the outside.

TECHNICAL FIELD

The present invention relates to electron beam irradiation apparatus andmethod, and more particularly to electron beam irradiation apparatus andmethod which are employed for irradiating combustion exhaust gasdischarged from thermal power stations or the like with an electron beamto remove toxic components from the exhaust gas.

BACKGROUND ART

As economy develops, more and more energy is demanded. Amidst thecontinuous growth of energy demand, energy source is still dependent onfossil fuels such as coal and petroleum. However, the harmful productsor pollutants generated by burning of fossil fuels are responsible forglobal pollution. It is considered that a global issue of the globalwarming and the acid rain caused by air pollution is attributed tocomponents such as SOx and NOx which are contained in combustion exhaustgas discharged from thermal power stations or the like. As a method forremoving toxic components such as SOx and NOx, there has been used amethod of irradiating combustion exhaust gas with an electron beam fordesulfurization and denitration (i.e. removing toxic components such asSOx and NOx).

In a flue gas treatment system for treating the combustion exhaust gasby employing an electrom beam, molecules such as oxygen (O₂) and watervapor (H₂O) in the combustion exhaust gas are irradiated with theelectron beam emitted from an irradiation window comprising a thin filmmade of Ti or the like to form radicals such as OH, O, and HO₂ havinghigh oxidizing strength. These radicals oxidize toxic components such asSOx and NOx to produce sulfuric acid and nitric acid as intermediateproducts. These intermediate products react with ammonia gas (NH₃)previously injected into the exhaust gas to produce ammonium sulfate andammonium nitrate which are recovered as materials for fertilizer.Therefore, such a system for treating exhaust gas can remove toxiccomponents such as SOx and NOx from the combustion exhaust gas andsimultaneously recover ammonium sulfate and ammonium nitrate as usefulby-products used for materials for fertilizer.

FIG. 3 shows an electron beam irradiation apparatus used for the aboveflue gas treatment system according to an example.

The electron beam irradiation apparatus 11 mainly comprises athermoelectron generator 12 comprising a filament or the like, anaccelerating tube 13 for accelerating electrons emitted from thethermoelectron generator 12, a focusing electromagnet 16 for controllinga diameter of the electron beam by applying the magnetic field to thehigh-energy electron beam formed in the accelerating tube 13, andscanning electromagnets 17, 18 for deflecting the electron beam in x andy directions by applying the magnetic field to the electron beam whosediameter has been controlled by the focusing electromagnet 16. The xdirection is a horizontal direction shown in FIG. 3, and the y directionis a direction perpendicular to the x direction and also perpendicularto the sheet surface of FIG. 3. A surrounding comprising a container 19and an irradiation window 20 is provided, and the interior of thesurrounding is kept under high vacuum condition in the range of1.33×10⁻³ to 1.33×10⁻⁴ Pa (10⁻⁵ to 10⁻⁶ Torr). The high-energy electronbeam formed by the accelerating tube 13 is deflected and scanned by thescanning electromagnets 17, 18 which apply the magnetic field to theelectron beam, and emitted through the irradiation window 20 into acertain range of an exhaust gas passage (not shown in FIG. 3) located atthe outside.

Thermoelectrons generated by the thermoelectron generator 12 comprisinga filament or the like are accelerated by high-voltage of about 800 kV,for example, in the accelerating tube 13 to cause a high-speed electronbeam to be formed. Then, a beam diameter of the electron beam iscontrolled by the focusing electromagnet 16 to thus form a linearelectron beam, having substantially the same diameter in a travellingdirection in an example shown in FIG. 3, which is then directed towardthe magnetic field formed by the scanning electromagnets 17, 18. Thefocusing electromagnet 16 comprises an electromagnet having aring-shaped coil disposed around an axis of the electromagnet, and formsa magnetic field which is symmetric with respect to the axis of theelectron beam. The beam diameter of the electron beam is controlled bymagnitude and direction of the magnetic field. In other words, focusingof the electron beam is controlled by magnitude and direction of themagnetic field. Therefore, direct current I₀ is supplied to the coil ofthe electromagnet, and the degree of convergence or divergence of theelectron beam is adjusted by magnitude of the direct current I₀.

The electron beam whose diameter has been controlled by the focusingelectromagnet 16 is scanned in the x and y directions by the scanningelectromagnets 17, 18. The scanning electromagnet 17 comprises anelectromagnet having a pair of poles for deflecting the electron beam inthe y direction, and the scanning electromagnet 18 comprises anelectromagnet having a pair of poles for deflecting the electron beam inthe x direction. By controlling magnitude and direction of currentsupplied to the coils of the scanning electromagnets 17, 18, angles ofdeflection in the x and y directions are controlled, and hence theelectron beam is scanned and the irradiation position of the electronbeam is controlled. In an example, the electron beam is scanned in the ydirection (latitudinal direction) using rectangular wave in the scanningelectromagnet 17, and the electron beam is scanned in the x direction(longitudinal direction) using sine wave in the scanning electromagnet18.

However, when the electron beam is scanned in the x direction by thescanning electromagnet 18, if the angle of deflection is large in thevicinity of maximum scanning positions A, B corresponding to bothscanning ends, the electron beam is deflected by the magnetic fieldproduced by the electromagnet, so that an angle of outgoing electronbeam differs according to an angle of incidence of the electron beam.Therefore, the electron beam converges at the irradiation windowportions A, B corresponding to the maximum scanning positions A, B dueto a lens effect created by a convex lens or the like. Specifically, asshown in the irradiation window portions A, B and C of FIG. 3, while thebeam diameter is about 10 cm, for example, at the central position C,the beam diameter is about 5 cm, for example, at the maximum scanningpositions A, B corresponding to both scanning ends. Thus, theirradiation area of the electron beam is remarkably converged at themaximum scanning positions A, B. The irradiation window 20 comprises athin film made of titanium (Ti), and hence if the electron beamconverges at the maximum scanning positions A, B or thereabouts, thenenergy density of the electron beam is increased thereat, causing damageto the irradiation window.

Further, areas where irradiation of the electron beam is not made areformed at the maximum scanning positions A, B or thereabouts, and hencetoxic components in the combustion exhaust gas cannot be sufficientlyremoved.

DISCLOSURE OF INVENTION

It is therefore an object of the present invention to provide electronbeam irradiation apparatus and method which can prevent an electron beamfrom being converged at a maximum scanning position and can stablyobtain an irradiation area having a uniform energy density whereirradiation of the electron beam is uniformly performed.

According to an aspect of the present invention, there is provided anelectron beam irradiation apparatus, comprising: an electron beam sourcefor emitting electrons; an accelerating tube for accelerating theelectrons emitted from the electron beam source; a focusingelectromagnet for controlling a diameter of an electron beam by applyinga magnetic field to an electron beam having a high energy formed in theaccelerating tube; an electromagnet for deflecting and scanning theelectron beam by applying a magnetic field to the electron beam; and anirradiation window for allowing the electron beam to pass therethrough;wherein the electron beam is focused at a focus point by the focusingelectromagnet so that the electron beam converges once and thendiverges, and then emitted through the irradiation window to theoutside.

According to the present invention, the electron beam converges once atthe focus point and then diverges, and then is emitted through theirradiation window to the outside. Therefore, the beam diameter of theelectron beam can be enlarged at the irradiation window. The beamdiameter of the electron beam which has focused tends to be larger atthe positions A, B having a large angle of deflection than that at thecentral position C. Thus, since the beam diameter of the electron beamis sufficiently enlarged at the maximum scanning positions (maximumangle of deflection), the convergence of the electron beam at theirradiation window portion can be prevented. Therefore, the irradiationdensity of the electron beam at the irradiation window portion can beuniformized to thus prevent the irradiation window portion from beingdamaged. Further, the electron beam is uniformly emitted through theirradiation window portion to allow combustion exhaust gas to beuniformly irradiated with the electron beam, thus removing toxiccomponents sufficiently from the exhaust gas.

It is desirable to position the prefucus point where the electron beamconverges once and then diverges at a location after the electron beampasses through the magnetic field for deflecting and scanning theelectron beam in a travelling direction of the electron beam. Thus, inthe case where the accelerating energy of the electron beam is so largeas to be about 800 kV and the velocity of the electron beam is close tothe light velocity, the electron beam becomes relativistic electron beam(REB). The present invention is applicable to such electron beam.Therefore, even if an angle of deflection is large, the beam diameterhaving a sufficient expansion at the irradiation window portion can beobtained.

It is desirable that the location of the focus point is adjusted bycontrolling current value supplied to the focusing electromagnet. Thus,by a relatively simple means for adjusting current value supplied to thefocusing electromagnet, the beam diameter having a sufficient expansionat the maximum scanning position on the irradiation window portion canbe obtained.

According to another aspect of the present invention, an electron beamirradiation method, comprising: controlling a diameter of an electronbeam having a high energy by applying a magnetic field to the electronbeam; deflecting and scanning the electron beam whose diameter has beencontrolled by applying a magnetic field to the electron beam with afocusing electromagnet; and emitting the electron beam through anirradiation window to the outside; wherein the electron beam is focusedat a focus point by the focusing electromagnet so that the electron beamconverges once and then diverges, and then emitted through theirradiation window to the outside.

With the above arrangement, even if the electron beam having a highenergy is scanned at a relatively large angle of deflection, theconvergence of the electron beam can be avoided, and irradiation of theelectron beam can be carried out in a uniform energy density over a widescanning area. Thus, the electron beam having a uniform energy densitycan be supplied to a relatively large irradiation area in an electronbeam irradiation apparatus for treating flue gas, or the like. Further,a relatively large angle of deflection of the electron beam can bepermitted to thus contribute to downsizing of the apparatus.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view showing the manner in which electron beam isconverged and diverged in an electron beam irradiation apparatusaccording to an embodiment of the present invention;

FIG. 2 is a schematic view of a flue gas treatment system in which theelectron beam irradiation apparatus shown in FIG. 1 is incorporated; and

FIG. 3 is a schematic view showing the manner in which electron beam isconverged and diverged in a conventional electron beam irradiationapparatus.

BEST MODE FOR CARRYING OUT THE INVENTION

An electron beam irradiation apparatus and method according to anembodiment of the present invention will be described below withreference to FIG. 1. Like or corresponding parts are denoted by like orcorresponding reference numerals throughout views.

As shown in FIG. 1, an electron beam irradiation apparatus 11 comprisesa thermoelectron generator 12, an accelerating tube 13, a focusingelectromagnet 16, and scanning electromagnets 17, 18, whereby theelectron beam is accelerated and scanned, and then emitted through anirradiation window 20 into a certain range of an exhaust gas passagelocated at the outside. This structure of the electron beam irradiationapparatus 11 is the same as that of the conventional apparatus. Further,the electron beam is accelerated to a high-speed by high-voltage ofabout 800 kV in the accelerating tube 13, and a beam diameter of theelectron beam is controlled by the focusing electromagnet 16, and thenthe electron beam whose diameter has been controlled is deflected andscanned by the scanning electromagnets 17, 18. This structure is alsothe same as the conventional apparatus. As a scanning width, thescanning length in the longitudinal direction (x direction) is in therange of 3 to 4 m, and the scanning length in the latitudinal direction(y direction) is in the range of 60 to 40 cm.

In the electron beam irradiation apparatus of the present invention,direct current I₀ supplied to the focusing electromagnet 16 is adjustedsuch that before the electron beam reaches the irradiation window 20,the electron beam is focused for thereby forming a sufficient expansionwhen the electron beam reaches the irradiation window 20. Specifically,by making the direct current I₀ supplied to the focusing electromagnet16 larger, as shown in FIG. 1, the focus of the electron beam isadjusted so that the electron beam E converges at a focus point F to amaximum degree after the electron beam passes through the main magneticfield produced by the scanning electromagnet 18. That is, the beamdiameter of the electron beam E becomes the smallest at the focus pointF. Therefore, after the electron beam E passes through the focus pointF, the electron beam E diverges to enlarge the beam diametersufficiently and reaches the irradiation window 20. Thereafter, theelectron beam E is emitted through the irradiation window 20 comprisinga Ti foil into a certain range of the exhaust gas passage, thusperforming desulfurization or denitrozation of the exhaust gas.

The beam diameter of the electron beam E is about 10 cm which is thesame as that of a linear electron beam obtained by the conventionalapparatus shown in FIG. 3. At the maximum scanning positions A, B on theirradiation window 20, the beam diameter of the electron beam E is equalto or larger than 10 cm. The position of the focus point F can beadjusted by magnitude of the direct current I₀, whereby the beamdiameter can be suitably adjusted at the irradiation window 20.

It is confirmed by various experiments that when flux density of themagnetic field produced by the scanning electromagnet 18 is zero, it ispreferable to position the focus point F at a location after theelectron beam passes through the magnetic field produced by the scanningelectromagnet 18. Accordingly, the entrance angle of the electron beaminto the magnetic field produced by the scanning electromagnets 17, 18is preferably controlled, and the outgoing angle of the electron beamout of the magnetic field is also preferably controlled aftercirculating motion therein. Therefore, the convergence and divergence ofthe electron beam can be carried out well at the focus point F.

According to the embodiment of the present invention, the energy ofelectron beam required for the electron beam irradiation apparatus inthe flue gas treatment system is about 800 kV, and the electron beam hasa high velocity close to the light velocity and is called relativisticelectron beam. The velocity of the electron beam is expressed by thefollowing formula (1): $\begin{matrix}{\frac{v}{c} = \left\lbrack {1 - \frac{1}{\left( {1 + \frac{ev}{m_{0}c^{2}}} \right)^{2}}} \right\rbrack^{\frac{1}{2}}} & (1)\end{matrix}$

where e represents electric charge of an electron equal to 1.6×10⁻¹⁹ C,m₀ represents electron mass equal to 9.1×10⁻³¹ kg, and c representslight velocity equal to 3×10⁸ m/s.

Therefore, an electron accelerated by high-voltage of 800 kV has avelocity v expressed using the light velocity (c) in the following:

V/c≈0.92

In the relativistic electron beam having a high velocity also, bypositioning the focus point F at the above-mentioned location, the beamdiameter of the electron beam at the irradiation window 20 can beenlarged to a suitable size. In the relativistic electron beam also, thebeam diameter of the electron beam is larger at the maximum scanningpositions A, B than that at the central position C.

Therefore, the convergence of the electron beam at the maximum scanningpositions A, B can be prevented, damage to the irradiation window can beprevented, and the electron beam having a uniform energy density can besupplied over the entire irradiation area.

FIG. 2 shows a flue gas treatment system incorporating the electron beamirradiation apparatus of the present invention in which flue gasdischarged from a fuel combustion facility such as a thermal powerstation is treated by irradiation of electron beam. As shown in FIG. 2,flue gas discharged from a thermal power station 21 which is a kind offuel combustion facility is cooled in a heat exchanger 22, and thenintroduced into a cooling tower 24. In the cooling tower 24, watersupplied from a pump 23 is sprayed by a fluid nozzle 26, and the sprayedwater is evaporated therein. The flue gas is cooled to a certain rangeof temperature in the cooling tower 24, and then the cooled gas isintroduced into a process vessel 25.

On the other hand, ammonia supplied from an ammonia supply equipment 29is mixed with air in a line mixer 30. The mixed gas and water suppliedfrom a water supply source (not shown) are mixed in a gas-liquid mixingroom of a two-fluid nozzle 31, and sprayed at the entrance of theprocess vessel 25. The mixture of the gas and water are irradiated withan electron beam from the electron beam irradiation apparatus 11 shownin FIG. 1.

In the above embodiment, the electron beam irradiation apparatus fortreating flue gas is described. However, the essence of the presentinvention lies in that the convergence of the electron beam caused by alarge angle of deflection when the electron beam is scanned can beavoided. Therefore, the present invention is applicable to variousapparatuses, including an electron beam welding apparatus, a scanningelectron microscope and the like, which utilize an electron beam.

As described above, according to the present invention, even if ahigh-energy electron beam is scanned at a relatively large angle ofdeflection, the convergence of the electron beam can be avoided, andirradiation of the electron beam can be carried out in a uniform energydensity over a wide scanning area.

INDUSTRIAL APPLICABILITY

The present invention relates to electron beam irradiation apparatus andmethod, and is applicable to a flue gas treatment system for treatingflue gas discharged from a fuel combustion facility such as a thermalpower station, an electron beam welding apparatus, or a scanningelectron microscope.

What is claimed is:
 1. An electron beam irradiation apparatus,comprising: an electron beam source for emitting electrons; anaccelerating tube for accelerating said electrons emitted from saidelectron beam source; a focusing electromagnet for controlling the sizeof a diameter of an electron beam by applying a magnetic field to anelectron beam having a high energy formed in said accelerating tube; anelectromagnet for deflecting and scanning said electron beam by applyinga magnetic field to said electron beam; and an irradiation window forallowing said electron beam to pass therethrough, wherein said electronbeam is focused at a focus point by said focusing electromagnet so thatsaid electron beam converges once and then diverges, and then emittedthrough said irradiation window to the outside.
 2. An electron beamirradiation apparatus according to claim 1, wherein said focus point ofsaid electron beam is positioned at a location after said electron beampasses through said magnetic field for scanning said electron beam. 3.An electron beam irradiation apparatus according to claim 2, whereinsaid location of said focus point is adjusted by controlling currentvalue supplied to said focusing electromagnet.
 4. An electron beamirradiation method, comprising the steps of: controlling the size of adiameter of an electron beam having a high energy by applying a magneticfield to said electron beam; deflecting and scanning said electron beamwhose diameter has been controlled by applying a magnetic field to saidelectron beam with a focusing electromagnet; and emitting said electronbeam through an irradiation window to the outside, wherein said electronbeam is focused at a focus point by said focusing electromagnet so thatsaid electron beam converges once and then diverges, and then emittedthrough said irradiation window to the outside.
 5. An electron beamirradiation method according to claim 4, wherein said focus point ofsaid electron beam is positioned at a location after said electron beampasses through said magnetic field for scanning said electron beam. 6.An electron beam irradiation method according to claim 5, wherein saidlocation of said focus point is adjusted by controlling current valuesupplied to said focusing electromagnet.
 7. An electron beam irradiationapparatus, comprising: an electron beam source for emitting electrons;an accelerating tube for accelerating said electrons emitted from saidelectron beam source; a focusing electromagnet for controlling adiameter of an electron beam by applying a magnetic field to an electronbeam having a high energy formed in said accelerating tube so as toenlarge said diameter of an electron beam at a maximum scanningpositions in relation to a central position of said electron beam; anelectromagnet for deflecting and scanning said electron beam by applyinga magnetic field to said electron beam; and an irradiation window forallowing said electron beam to pass therethrough, wherein said electronbeam is focused at a focus point by said focusing electromagnet so thatsaid electron beam converges once and then diverges, and then emittedthrough said irradiation window to the outside.
 8. An electron beamirradiation apparatus according to claim 7, wherein said focus point ofsaid electron beam is positioned at a location after said electron beampasses through said magnetic field for scanning said electron beam. 9.An electron beam irradiation apparatus according to claim 8, whereinsaid location of said focus point is adjusted by controlling currentvalue supplied to said focusing electromagnet.
 10. An electron beamirradiation method, comprising: controlling a diameter of an electronbeam having a high energy by applying a magnetic field to said electronbeam so as to enlarge said diameter of an electron beam at a maximumscanning positions in relation to a central position of said electronbeam; deflecting and scanning said electron beam whose diameter has beencontrolled by applying a magnetic field to said electron beam with afocusing electromagnet; and emitting said electron beam through anirradiation window to the outside, wherein said electron beam is focusedat a focus point by said focusing electromagnet so that said electronbeam converges once and then diverges, and then emitted through saidirradiation window to the outside.
 11. An electron beam irradiationmethod according to claim 10, wherein said focus point of said electronbeam is positioned at a location after said electron beam passes throughsaid magnetic field for scanning said electron beam.
 12. An electronbeam irradiation method according to claim 11, wherein said location ofsaid focus point is adjusted by controlling current value supplied tosaid focusing electromagnet.